Free-radical photopolymerization has been used to prepare polymetric materials including films, adhesives, coatings and composites. The process has the advantages of low energy demand, rapid and readily controllable reaction kinetics, excellent mechanical properties and the versatility available with a broad array of monomers. It has been reported that visible light activated three-component initiator systems (utilizing a photosensitizer, an electron acceptor, and an electron donor) produce enhanced rates of polymerization, higher sensitivity and higher conversions than the visible light-activated two-component initiator systems that include a photosensitizer and an electron donor (Chesneau, E., Fouassier, J. P.; Angew. Makromol Chem. (1985), 135, 41; Kawabata M.; Takimoto, Y.; J. Photpolymer Sci Technol. (1991) 3, 147; Hoechst, A. G.; Japan Kokai (1990), 02, 127,404; Kumar, G. S.; Neckers, D. C., Marcomolecules (1991) 24, 4322; Fouassier, J. P., Morlet-Savary, F R.; Yamashita, K.; Imahashi, S., J. of Applied Polymer Science (1996) 62, 1877). Several effective three-component initiator systems have also been reported (U.S. Pat. No. 4,735,632, U.S. Pat. No. 4,828,583, U.S. Pat. No. 6,017,660, U.S. Pat. No. 6,043,295).
Radical-based photopolymerization is characterized by a rapid cessation of polymerization when the photo-curing light source is extinguished. To maintain the active polymerization to completion, continuous initiation is required because radical based active centers have short propagating times due to highly efficient termination reactions. In contrast, cationic photopolymerization allows significant dark curing because of long active center lifetimes. But, compared with radical polymerizations, there are few monomers compatible with the cationic curing process. This dark curing behavior is one of the distinct differences between cationic and free-radical photopolymerization, and the prospect of removing this dark cure limitation from radical-based polymerizations with conventional monomers is highly significant because effective dark curing would be useful to reduce processing times and lower initiator concentrations. as well as to achieve photo-curing in shadow regions, enhancing the depths of cure and for photo-curing pigmented or highly-filled systems using radical polymerizations.
There are few examples of free-radically polymerizable monomers that exhibit considerable dark cure potential (Kilambi, H.; Reddy, S. K.; Schneidewind, L; Stanswbury, J. W.; Bowman, C. N. Copolymerization and dark polymerization studies for photopolymerization of novel acrylic monomers. Polymer (2007) 48: 2014-2021). These are (meth)acrylate monomers that are characterized by a number of unusual properties such as hyper-reactivity and significant formation of crosslinks from mono-vinyl polymerization in addition to substantial dark cure potential with conventional photo-initiating systems. However, these monomers are not commercially available and their unique behavior is dramatically different from that of conventional (meth)acrylate monomers, such as HEMA and HEA, that are more frequently used in polymerization reactions.
Therefore, there remains a need in the art for a method for extensive dark curing from a visible-light initiated controlled radical polymerization of commonly used monomers.